The invention relates to a vehicle driving arrangement having at least two driven wheels the driving torques of which can be individually regulated by a control device, the control device being connected to a sensor arrangement comprising at least one sensor.
A vehicle driving arrangement of that kind is known from DE 196 38 421 A1. The vehicle described therein is driven hydraulically. The control device is connected to several sensors. One group of sensors ascertains the respective speeds of the wheels. A further sensor establishes whether the vehicle is moving over an inclined surface having an angle of inclination greater than 7xc2x0. By comparing the rolling speeds of the wheels, which are found from the dimensions of the wheels and the speeds of rotation, wheel slip is calculated. If the wheel slip becomes too great, that is to say, if the speed of one wheel greatly differs from the speed of another wheel, the displacement of the hydraulic motor of the wheel having the highest wheel speed is reduced. The sensor that detects movement over an inclined surface is used to prevent power reduction of a drive motor in certain circumstances, namely when the vehicle is travelling uphill or downhill.
A similar arrangement is known from U.S. Pat. No. 5,201,570. Therein, four wheels are driven by hydraulic motors. The wheel speed of each wheel is monitored. If the speeds differ greatly, then the xe2x80x9cfastestxe2x80x9d wheel is braked.
Such methods of preventing slip are referred to as reactive processes. Slip has to have occurred in the first place, therefore. When such slip has been detected, the driving power of the wheel is reduced so that the wheel changes over from sliding friction to rolling friction again.
Various kinds of reactive processes are known from the automobile industry and are used in anti-slip systems. In some cases, the wheel that is assumed to be spinning is also braked.
Such driving arrangements have proved successful in many cases. They suffer from the disadvantage, however, that wheel slip has to occur in the first place before it can be remedied. Those driving arrangements are not suitable, therefore, for vehicles that must not damage the ground on which they are travelling. A typical example of such a situation arises in the case of self-propelled lawn mowers as are used, for example, for golf courses. A spinning wheel will damage the grass turf. The purpose of the lawn mower, to care for the lawn, will thus be reversed.
The problem underlying the invention is to keep the risk of wheel slip small.
That problem is solved in the case of a driving arrangement of the kind mentioned at the beginning by means of the fact that, using a signal of the sensor arrangement, the control device continuously ascertains a maximum torque for each wheel, continuously ascertains the driving torque of each wheel and reduces the driving torque by the appropriate amount when the driving torque ascertained no longer complies with a predetermined limit of difference from the maximum torque.
Driving torque is to be understood herein to mean both a moment that serves to propel the vehicle and a moment that serves to brake the vehicle. The latter case occurs, for example, when the vehicle is rolling downhill and is to be braked by the engine.
The control device continuously ascertains for each wheel, therefore, a maximum torque, using signals from the sensor arrangement. The maximum torque is the moment at which the wheel spins (when driven) or locks (when braked). A slip condition of that kind occurs when the force parallel to the ground at the bearing point between the wheel and the ground (hereinafter xe2x80x9cdriving forcexe2x80x9d) is greater than the adhesive force which can be calculated, for example, from the product of the coefficient of friction and the normal force. The normal force is known in principle. It is determined mainly by the weight of the vehicle (with driver and fittings). It changes during operation, however, owing to various circumstances. Those circumstances can be detected by the sensor arrangement, with the result that the maximum permissible torque can be ascertained with greater reliability. If care is taken that the driving torque acting on the individual wheel is kept smaller than the maximum permissible moment, then slipping does not occur at all in the first place. Damage to the ground is therefore reliably avoided.
In a preferred embodiment, provision is made for the sensor arrangement to have an inclination sensor. A decisive factor in the determination of the normal force from the weight is the inclination of the vehicle relative to the direction of gravity. The more inclined is the. vehicle, the smaller becomes the normal force towards the ground and the greater becomes the risk of slip. In addition, when the vehicle is inclined, a different weight distribution over the individual wheels, and hence a change in the normal force distribution over the wheels, is also produced. That change also can be taken into consideration, so that, for example, the wheels that are lower down receive a greater driving power than do the wheels that are higher up.
It is preferred in that connection for the inclination sensor to ascertain the inclination parallel to the longitudinal direction of the vehicle and the inclination transverse to the longitudinal direction of the vehicle. In most cases, the vehicle will be inclined both longitudinally and transversely. By ascertaining the two angles of inclination separately, however, the control can be simplified and improved.
The sensor arrangement preferably has a normal force sensor. The normal force is determined in principle, as mentioned above, according to the weight of the vehicle which is distributed over the individual wheels in accordance with a specific ratio system. That weight can change, however. For example, various drivers with differing body weights can be used. The vehicle consumes fuel, as a result of which its weight decreases during operation. If the vehicle is spreading any products, for example is applying fertilisers or sowing grass seed, the weight of the vehicle similarly changes. By means of the normal force sensor it is possible to ascertain that change during operation or between individual phases of operation.
In one advantageous embodiment, the normal force sensor is in the form of a load cell. In that case, it actually weighs the vehicle. The weighing can be limited to when the vehicle is standing on a horizontal plane surface if the inclination is later taken into consideration.
In another or additional embodiment, the normal force sensor can be in the form of a pressure transducer which ascertains the tyre pressure of the driven wheel. The tyre pressure is also a measure of the loading of the tyre and hence of the force acting in the vertical direction. When the vehicle is standing on a horizontal plane surface, the tyre pressure is directly a measure of the normal force. In the case of inclined ground, the normal force has to be converted accordingly.
Finally, the normal force sensor can also monitor the spring behaviour of a vehicle spring system. The more the vehicle is loaded, the more it deflects the spring system. Evaluation of the spring characteristic allows information to be gained, at least indirectly, on the vehicle weight, and hence also on the normal force on the driven wheel.
Preferably, the control device is connected to a memory in which vehicle data are filed. Those data can include, in addition to the standard vehicle weight, also details of the rolling resistance of the vehicle, the tractive force required for any trailer and/or towed work implement, such as, for example, mowing apparatus or the like. The memory can also contain information on how specific interventions in the driving power affect the individual motors.
In that connection it is preferred that, for predetermined vehicle data-sensor signal combinations, the corresponding maximum torques of the respective wheels are filed in the memory in the form of a table. For example, the normal force of each individual wheel as a function of the angle of inclination can be filed in the table. The table allows more rapid access to the results than does a calculation.
In addition, the control device can be connected to an input device by means of which ambient and/or operating parameters can be specified. The condition of the ground, for example, can be termed an ambient parameter. In the case of a lawn, a rough distinction according to normal, wet and dry could be made. As an operating parameter, it is possible to specify whether a trailer has to be towed or other work implements are attached.
Preferably, when the control device reduces the driving power of one wheel, it increases the driving power of at least one other wheel. In that case, the overall driving power of the vehicle remains constant, so that the driver does not even notice a change in the drive distribution in most cases.
It is especially preferred in that connection if the control device produces a warning in an operating situation in which a reduction of the driving powers of all the driven wheels is necessary. The driver can then decide what steps to take. For example, he can specify a different coefficient of friction so that the maximum permissible torque is calculated differently. Since in that case, however, there is an increased risk of wheel slip, the driver must proceed very cautiously. Increasing of that coefficient of friction can also be carried out by the system automatically. In that case, the driver must similarly be given a warning.
Preferably, the control device is connected to a steering angle sensor. The control device is therefore able to ascertain the angular position of steered wheels. As soon as a wheel has been turned, that is to say, is no longer rolling in the principal direction of movement of the vehicle, it has a higher travelling resistance, on the one hand, and has a greater tendency to slip, on the other hand. Using the steering angle sensor, that effect can be taken into consideration.
It is also advantageous if the vehicle is equipped with auxiliary-power-assisted steering which is connected to the control device, the control device taking the steering behaviour of the steered wheel into consideration when determining the maximum torque. In addition to or instead of the steering angle sensor, the dynamic behaviour of the wheel during a steering movement can also be taken into consideration in that manner. During the steering movement, the maximum torque is usually reduced.